1. Field of the invention
The present invention relates to expansion turbine having a variable nozzle mechanism used in large refrigerating machines such as helium refrigerating machine. Priority is claimed on Japanese Patent Application No. 2007-89477, filed Mar. 29, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
Expansion turbines have been used conventionally to enhance the efficiency of refrigerating machines. To regulate the flow rate of gas introduced into such an expansion turbine, as shown in FIG. 6, expansion turbines using variable nozzle mechanism 10 are popularly used (for example, refer to the Japanese Unexamined Patent Application, First Publication No. 2001-132410.)
This variable nozzle mechanism 10 comprises a nozzle member 14 used to change the throat area of very low temperature gas introduced into a turbine impeller 12, and a driving member 16 used to drive the nozzle member 14. The nozzle member 14 is built into an adiabatic expansion device 20 located in a vacuum container 18. The driving member 16 is disposed outside the vacuum container 18 so as to not expose it to low temperatures and thereby ensure mechanical reliability.
As shown in FIG. 6, the nozzle member 14 and the driving member 16 are connected to each other by a thin cylindrical member 22 coaxial with a turbine impeller 12. The nozzle member 14 is driven by the oscillation of the cylindrical member 22 around the axial center C of the turbine impeller 12.
The nozzle member 14 is disposed to surround the turbine impeller 12, and comprises a plurality of movable nozzle plates 14a each of which is oscillatably connected to and supported by the adiabatic expansion device 20 through support pin 24, and a drive disc 28 connected to the inside end of the cylindrical member 22 and engaged with each movable nozzle plate through drive pin 26.
These are pressed against the adiabatic expansion device 20 after receiving a biasing force in the direction of the axial center C by a retaining spring 30 provided on the drive side, so that no clearance occurs between the nozzle member 14, the drive disc 28 and the adiabatic expansion device 20, thereby preventing leakage of gas on the nozzle face. In this manner, degradation in performance of the expansion turbine is prevented. Moreover, the driving member 16 comprises a rotating drive device 36 such as a pulse motor for driving an oscillatable gear 32 with center as the axial center C of the turbine impeller 12 connected to the outside end of the cylindrical member 22.
This variable nozzle mechanism 10 oscillates the cylindrical member 22 about the axial center C of the turbine impeller 12 by driving the rotating drive device 36, oscillates the drive disc 28, oscillatably drives the movable nozzle plate 14a about the support pin 24 taken as the center, and changes the angle of the movable nozzle plate 14a. In this manner, by continuously changing the throat area of the variable nozzle, the flow rate of gas passing through is regulated.
In such a conventional expansion turbine, the turbine impeller 12 is rotatably driven during adiabatic expansion of very low temperature gas. The pressure of gas on the exit side 15b of the nozzle member 14 on the turbine impeller 12 side is low, while the pressure of gas on the entrance side of the nozzle member 14 is high.
This gas enters the boundary surface of the drive disc 28 adjacent to the nozzle member 14 and the adiabatic expansion device 20, and exerts pressure on each boundary surface. That is, the high pressure gas on the entrance side 15a of the nozzle member 14 is made to enter the small clearance 1 between the cylindrical member 22 and the casing 19 of the vacuum container 18. The flow in the axial direction of this high pressure gas is obstructed by sealing member 25 such as the O-ring seal provided on the outer peripheral surface of body 23 of the cylindrical member 22.
On the other hand, the low pressure gas on the exit side 15b of the nozzle member 14 passes through the small clearance between insulating material 17 and the drive disc 28, and goes around the clearance 3 between the rear face (outside end face) of the drive disc 28 and the insulating material 17, applies pressure on the clearance 4 between the inner peripheral surface of the cylindrical member 22 and the insulating material 17, the outside end face 5 of outer flange 21, around the gear 32, the clearance 6 between the inside end face of the outer flange 21 and the casing 19, and the clearance 7 between the outer peripheral surface 23 of the cylindrical member 22 and the casing 19, and its flow in the axial direction is obstructed by the sealing member 25. Thus, the action of pressure due to gas is applied on each member.
In expansion turbines using the conventional variable nozzle mechanism 10 as mentioned above, the driving member 16, the cylindrical member 22, the gear 32 and the drive unit 40 including the rotor shaft 38 are configured to be removed as an integral body from the adiabatic expansion device 20 in the vacuum container 18. The nozzle member 14 is left behind in the adiabatic expansion device 20.
Incidentally, an axial outwardly directed force acts on the drive disc 28 as a result of the action of pressure by gas on each member in the expansion turbine using the conventional variable nozzle mechanism 10 mentioned above. That is, high gas pressure acts on the face 8a on the entrance side 15a of nozzle member 14 in contact with high pressure gas outwardly in the radial direction in the inside end face 8 of the drive disc 28, and low gas pressure acts on the face 8b on the exit side 15b of nozzle member 14 in contact with low pressure gas inwardly in the radial direction. On the other side, the pressure of low pressure gas around the back of the drive disc 28 acts on the face 9 of the outside end of the drive disc 28.
For this reason, the axial components of pressure of low pressure gas acting on the inside end face 8b and the outside end face 9 inwardly in the radial direction of the drive disc 28 cancel out each other, while the axial components of pressure of high pressure gas acting on the inside end face 8a outwardly in the radial direction and of pressure of low pressure gas acting on the outside end face 9 cannot cancel each other because the component on the high pressure side is greater. The result is that the drive disc 28 is pressed outward in the axial direction because of the difference in high pressure and low pressure.
The drive side face of the nozzle member 14 is connected so as to come into contact with the inside end face 8 of the drive disc 28. Accordingly, the force pressing the drive disc 28 outwardly in the axial direction acts so as to lift the nozzle member 14 outwardly in the axial direction. For this reason, a clearance is generated between the nozzle member 14 and the adiabatic expansion device 20. This led to gas leak from the clearance, which sometimes degraded the turbine performance.
To prevent such clearances, a retaining spring 30 is generally used to provide the resisting force to the lifting of the nozzle member. However, the force due to the difference in pressure is extremely large. For instance, if the gas pressure on the entrance side 15a of the nozzle member 14 is 2 MPa, and the gas pressure on the exit side 15b of the nozzle member 14 is 1 MPa, then the difference in pressure becomes 1 MPa. For this reason, a retaining spring 30 that could support a very large force in the axial direction equivalent to a maximum of 400 kgf (3.92 kN) to resist the force lifting the nozzle member 14 became necessary.
Moreover, in this case, the nozzle member 14 has to be driven while the keeping the resisting force acting to limit the difference in pressure; so a very large driving torque was necessary. This made it necessary to use a very large device and to adequately consider the strength of components during design, and thus required more labor and effort.
For this reason, development of an expansion turbine was demanded that could reduce the force lifting the nozzle member and at the same time, have no adverse effect on turbine performance.
The present invention considers the circumstances mentioned above, and has the object of offering an expansion turbine having a variable nozzle mechanism of simple configuration that avoids the action of axial force due to difference in pressure of gas in the drive unit of the nozzle member, does not require a very large suppressing force, does not require special considerations related to component strength and drive torque, and moreover, does not have any adverse effects on the original performance of the expansion turbine.